Measuring instruments with ultrasonic sensors are used in many applications. For instance, ultrasonic sensors are used as transmitters and/or receivers in fill level measuring instruments for distance measurement on the principle of echo sounding.
With fill level measuring instruments, among other things fill levels of a product in a container, for instance, or in an open channel can be measured. A signal emitted by the ultrasonic sensor, for instance a brief ultrasonic wave pulse, is transmitted in the direction of the product and is reflected from the surface of the product. The transit time of the pulse from the sensor to the surface and back is ascertained, and from that the fill level or fill height is determined.
Such measuring instruments are used in many branches of industry, such as in the food industry, in the fields of water supply and wastewater treatment, and in chemistry.
Depending on the transmission power and range desired, ultrasonic sensor with different resonant frequencies are employed. Typically, the resonant frequencies are in a range from 1 kHz to 200 kHz.
Measuring instruments therefore advantageously have an electronic measuring instrument unit, to which various ultrasonic sensors can be connected.
Before the measuring instrument can be put into operation, it must be ascertained which ultrasonic sensor is connected to it. This is preferably done automatically, by equipping the measuring instrument with an apparatus for detecting which ultrasonic sensor is connected, and this apparatus performs that task.
In conventional measuring instruments, ultrasonic sensors are therefore equipped with an identification resistor. When the ultrasonic sensor is connected, the identification resistor is simultaneously connected to a resistance measuring circuit, which from the resistance of the identification resistor determines which ultrasonic sensor is connected.
However, this method is quite complicated. The resistor must first be furnished, and two connection lines must be provided for the identification resistor and connected.
Another factor is that ultrasonic sensors without an identification resistor cannot be detected.